Ground Loop

su966 · 16 Nov 2016

We have a projector that is installed in the assembly hall. A long VGA cable has been run which may be 30 to 40 meters long. At the other end of this cable we connect our laptop. The video quality is fine. When we plug in any speaker to the laptop there is a very strong noise. I have checked there is voltage between VGA shield cable and the power socket ground at the laptop end. It is small voltage. There is resistance between VGA shield and Power ground at the laptop end. We use an isolator between laptop and speaker. It has solved the problem partially. But there is still noise.

Can not figure out how I can solve this problem. Any suggestion will be highly appreciated.

Sam Gangee · 16 Nov 2016

I don't understand what you mean by "isolator".

A laptop computer can't drive a loudspeaker so you must be using an amplified speaker system - presumably stereo. The audio signal will be at "line level" and requires a screened cable. The quality of cable makes a difference. The majority of "screened" cables with 3.5mm mini jack plugs are not actually screened. See example:-
http://www.satcure.co.uk/accs/page18.htm#M233

The problem you describe is common and the simple solution is usually to disconnect the laptop charger.

ninjacat · 16 Nov 2016

I presume you mean a DI or isolating transformer. If so, maybe it's gain structure and you are amplifying the inherent noise floor from the laptop? Another good way of getting audio out of a laptop into an amplifier is using a USB audio interface as on board audio out of laptops can be anything from decent to abysmal. Focusrite are a good entry level for decent sound. I use an M Audio box if I have to put my laptop through a system for work (events, corporate presentations etc).

Sam Gangee · 17 Nov 2016

The type of DI that you have is not suitable for an unbalanced input.

See the last couple of minutes of this video:

su966 · 17 Nov 2016

My problem is very common as per picture above. There is a ground loop current that is flowing. Earth at two separate places are not identical. Means they are not 0V at both places.

Someone may have solved this problem.

ninjacat · 17 Nov 2016

as Sam says, unplug the charger from the laptop. Does the noise go? Have you tried an extension lead from same supply as sound desk/ amplifier to projector? Have you got a vga humbucker you could put in line?

jackthom · 10 Dec 2016

A stereo isolation transformer unit would be the best solution. Then the inputs and outputs are completely isolated from each other and there's no circuit for the ground loop current to flow through.

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Ground Loop

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